NativeAOT support: get to zero warnings

Right now, when publishing a NativeAOT app using SqlClient, many warnings are emitted. To properly support NativeAOT, SqlClient will need to get to a place where no warnings are emitted, so the user can be 100% confident that the application will function correctly.

Note that this is related, but not the same as #1942, which is about reducing the binary size. However, the same technique can be used for both: if an entry point (e.g. slim data source builder) is introduced, which by default doesn't include functionality that is large and/or unsafe for NativeAOT, then programs not explicitly bringing in that functionality won't get warnings etc.